Perfected process for the preparation of stable nitroxide radicals

A process for preparing a stable nitroxide radical compound of formula (I):

• wherein R1, R2, R3 and R4, which may be the same or different, represent alkyl or isoalkyl groups having from 1 to 8 carbon atoms,
• wherein said process comprises:
• a) preparing an N-benzyl-1,1,3,3-tetra-alkylisoindoline intermediate in a reaction mixture by reacting in the presence of an aromatic solvent an N-benzylphthalimide with a Grignard reagent formed by reacting in the presence of a methyl tertiary butyl ether solvent an alkyl halide with magnesium;
• (b) removing said methyl tertiary butyl ether solvent by continuously distilling during said preparing;
• (c) purifying said reaction mixture by oxidizing impurities present within said reaction mixture with air while stirring for a period of 3 to 5 hours to produce insolubilized oxidized impurities;
• (d) removing said insolubilized oxidized impurities from said reaction mixture by filtering said reaction mixture on a celite panel to obtain a purified filtrate;
• (e) isolating said N-benzyl-1,1,3,3-tetra-alkylisoindoline intermediate by removing said aromatic solvent from said purified filtrate;
• (f) forming a 1,1,3,3-tetra-alkylisoindoline by subjecting said isolated N-benzyl-1,1,3,3-tetra-alkylisoindoline intermediate to hydrogenolysis in the presence of hydrogen and a supported palladium catalyst, at room temperature and at atmospheric pressure;
• and
• (g) yielding a stable nitroxide radical of formula (I) by oxidizing said 1,1,3,3-tetra-alkylisoindoline with hydrogen peroxide in the presence of a catalyst selected from polymolybdic acids, polytungstic acids, and salts thereof, optionally incorporating an element selected from phosphorus and silicon.